Disposal of industrial waste products such as fly ash, bottom ash, economizer ash, cement kiln dust (CKD), steel slag, and blast furnace slag (i.e., iron slag) is problematic. These waste products include significant amounts of calcium, alumina, and silica and are produced in enormous quantities.
Coal combustion by-products such as fly ash, bottom ash, and economizer ash are plentiful and pose a persistent disposal problem. The chemical content and particle size of fly ashes vary widely in accordance with the source of the coal, the fineness to which it is ground and the furnace within which it is burned.
ASTM C 618-85 Standard Specification for Fly Ash and Raw Calcined Natural Pozzolan for Use as a Mineral Admixture in Portland Cement Concrete, pp. 385-388 (1985) has classified fly ash into two classes, class C and class F. Class C fly ash has hardening properties due to its cementitious mineralogy. Class C fly ash typically contains between 70% and 50% by weight of silica, alumina and ferric oxides. Class C fly ash is usually high in calcium and is produced as a by-product of the combustion of lignite or subbituminous coal. Class F fly ash has non-hardening properties due to its pozzolanic mineralogy. Class F fly ash typically contains more than 70% by weight of silica, alumina, and ferric oxides.
According to ASTM C618, the chemical requirements to classify fly ash are as shown in Table 1.
TABLE 1Chemical Requirements for Fly Ash ClassificationFly Ash ClassPropertiesClass FClass CTotal of Silicon dioxide70.050.0(SiO2), aluminum oxide(Al2O3), and iron oxide(Fe2O3) min %Sulfur trioxide (SO3) max %5.05.0Moisture content, max %3.03.0
Class F fly ash is produced from burning anthracite and bituminous coals. Some Texas lignites also produce class F fly ash. This fly ash has siliceous or siliceous and aluminous material, which itself possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperature to form cementitious compounds. Class C fly ash is produced normally from lignite and subbituminous coals and usually contains significant amount of combined calcium (reported as CaO). This class of fly ash, in addition to having pozzolanic properties, also has some cementitious properties (ASTM C 618-99).
Bottom ash results from the burning of any type of coal. Bottom ash is a sandy, granular or clinkery residue consisting of mostly silicon dioxide and aluminum oxide. Bottom ash can be somewhat pozzolanic, but does not itself harden on contact with water. However, bottom ash in combination with class C fly ash can form chemical bonds that quickly unite and harden. Bottom ash typically collects in the bottom of furnaces and boilers where coal is burned and quenched with water before removal. Reactive cementitious compounds present in the bottom ash usually react during the quenching process.
Bottom ash has been characterized as a dark, gray, granular, porous predominantly sand size minus 12.7 mm (½ in.) material that is collected in a water filled hopper at the bottom of the furnace. Typically, when a sufficient amount of bottom ash drops into the hopper, it is removed via water jets and the like and conveyed by a sluiceway either to a decant basin for dewatering, crushing and stockpiling.
Typical properties of bottom ash are as follows:
TABLE 2Bottom Ash Particle Size DistributionBottom AshLocationLocationLocationSieve Size#1#2#338mm (1½ in)1009910019mm (¾ in)100951009.5mm (⅜ in)10087734.75mm (No. 4)9077522.36mm (No. 8)8057321.18mm (No. 16)7242170.60mm (No. 30)6529100.30mm (No. 50)561950.15mm (No. 100351520.075mm (No. 200)941
TABLE 3Typical Physical Properties of Bottom AshPropertyBottom AshSpecific Gravity(6)2.1-2.7Dry Unit Weight(6)720-1600 kg/m3(45-100 lb/ft3)Plasticity(6)NoneAbsorption(4)0.8.-2.0%
Bottom ash is composed primarily of silica, alumina, iron, and calcium with smaller percentages of magnesium, sulfates and other compounds. Table 4 presents a chemical analysis of selected samples of bottom ash from different coal types and regions.
TABLE 4Chemical Composition of Selected BottomAsh Samples (Percent by Weight)(4)Ash Type:Bottom AshCoal Type:BituminousSubbituminousLigniteLocationWest VirginiaOhioTexasSiO253.645.947.145.470.0Al2O328.325.128.319.315.9Fe8O35.814.310.79.72.0CaO0.41.40.415.36.0MgO4.25.25.23.11.9Na2O1.00.70.81.00.6K2O0.30.20.2—0.1
Economizer ash is, as the name suggests, removed from the economizer section of the boiler and typically has high calcium oxide content, high unburned carbon content and comprises course particles that are prone to forming clinkers.
A chemical analysis of one sample of economizer ash is shown in Table 5.
TABLE 5Typical Economizer AshConstituentWt %SiO233Al2O318Fe2O37SO32CaO24Na2O1MgO4
Other compounds such as TiO2, K2O, and P2O5 etc. are present so as to constitute 100 wt %. Physical particle sizes for this economizer ash are shown in Table 6.
TABLE 635% retained on 50 mesh screen36% of sample between 50 mesh and 200 mesh29% of sample was less than 200 mesh in size.Bulk density = 74 lbs. per ft3
Steel slag and blast furnace slag (i.e., iron slag) are other industrial waste by-products that include significant amounts of calcium, alumina, and silica components. In both steel and iron manufacturing processes, the molten metal collects in the bottom of the furnace with liquid iron or steel slag floating on the pool of the desired metal. The slag is collected and presents a disposal problem. It consists primarily of silica and alumina compounds combined with calcium and magnesium oxides.
Similarly, cement kiln dust comprises significant calcium, alumina, and silica components and is an industrial by-product in need of disposal alternatives. Typically, the CKD is collected from both the exit and feed end typically of a rotary cement kiln. The fine particulates are usually collected in high efficiency dust collectors such as fabric filters or electrostatic precipitators.
CKDs are composed of a mixture of sulfates, chlorides, carbonates, and oxides of sodium, potassium, and calcium in combination with quartz, limestone, fly ash, dolomite, feldspars and iron oxides, glasses of silicon dioxide, aluminum oxide, iron oxide, and certain cement compounds.
A need in the art therefore exists for commercially viable products and methods in which waste product materials may be advantageously used so as to minimize disposal concerns. An even more specific need in the art exists for commercially viable products and methods that incorporate coal combustion by-products such as fly ash, bottom ash, and/or economizer ash.
A product and method of use thereof by which 100% of the components are formed from such coal combustion by-products is even more desirable.